Softer insecticides reduce pests in winter vegetables

The U.S. pesticide industry has evolved over the decades with products which are safer for the environment and more efficacious on crop pests and diseases.
Today, the vast majority of insecticides, if not all, have either translaminar (leaf penetration) and/or true systemic (root uptake) activity.
Insecticide usage is down significantly in winter vegetable production in the Western low-desert region – from an average of 12 sprays in 1991 to about 4.5 sprays in 2011 – almost a 70 percent reduction.

The U.S. pesticide industry has evolved over the decades with products which are safer for the environment and more effective on crop pests and diseases.

The first generation of insecticides included highly toxic compounds to safeguard crops. The second generation — synthetic organic compounds — brought safer and more effective means for control.

Old timers in agriculture likely remember organophosphate and carbamate insecticides in the 1940s and 1950s followed by the pyrethroids in the 1970s. Back in the day, most chemistry controlled a broad spectrum of insects through contact activity or ingestion.

Today, the vast majority of insecticides, if not all, have either translaminar (leaf penetration) and/or true systemic (root uptake) activity.

“These developments have made a huge positive difference in insect control,” says John Palumbo, University of Arizona entomologist based at the Yuma Agricultural Center.

“Today, we’re using more soft chemistry than harder chemistry.”

Palumbo addressed past, present, and future insecticidal issues in desert-grown winter vegetables during the Bayer CropScience Vegetable and Citrus Consultants Meeting held in San Diego, Calif., in late July.

Today, insecticide usage is down significantly in winter vegetables. In 1991, Palumbo says growers and pest control advisers (PCAs) applied an average of 12 insecticide sprays per leafy vegetable crop. The number dropped to about eight sprays in 1996. Last year, the number plunged to an average of 4.5 sprays. This represents an almost 70 percent reduction in insecticide sprays over the last 20 years.

“Newer, green products are more efficacious compared to the older products,” Palumbo said.

The use of broader, toxic chemistry has declined due to the loss of product registrations in several types of lettuce. In addition, chemical companies brought new compounds to market in the early to mid-1990s, including the products Confirm, Success, and Admire. More recently, Movento and the diamides have allowed farmers to gain even better insect control with fewer applications.

The diamide class of chemistry “has risen to the top,” says Palumbo, as highly effective insect control products in veggies.

In desert vegetable fields, the top insects which prefer to dine on fall lettuce include whitefly, aphids, and the Leptidopterous pest complex, including the beet armyworm (Spodoptera exigua), cabbage looper (Trichoplusia ni), and corn earworm (Heliocoverpa zea).

Anyone who lived in the arid, low-desert region of southern California and Arizona 20 years ago will remember huge white swarms of Bemisia whitefly which resembled a Biblical pestilence outbreak.

To combat the whitefly epidemic, Arizona and California produce growers, in cooperation with Miles Inc., requested and received a Section 18 emergency use for its new insecticide Admire 2F (imidacloprid-based) to protect vulnerable winter vegetables in Yuma and Imperial counties. Today, Miles Inc. is called Bayer CropScience.

Successful whitefly control was achieved with a 16-ounce rate per acre application of Admire 2F applied by pre-plant injection. The dose provided immediate plant protection, plus 45-60 day residual control.

Over the last 20 years, Palumbo has conducted numerous insecticide trials in vegetables to compare insecticide product efficacy against whitefly. Over the last decade, he has conducted meetings and placed phone calls with PCAs and growers on the insecticides used for insect control plus crop losses.

Although imidacloprid may not provide the same level of control it did 20 years ago, UA research along with reports from PCAs and growers suggest that excellent whitefly control can be achieved with imidacloprid products when used at high rates per acre — for example 10.5 ounces of Admire Pro.

Palumbo said, “Imidacloprid-based products are the most cost effective soil insecticides on the market for whitefly control on lettuce and broccoli.”

Product placement is crucial at the root and secondary root levels which can provide 40 days of whitefly residual control.

“The fact is you can come in over the crop top with Movento, even by air, and still pick up a lot of immature whitefly populations at the basal part of the plant,” Palumbo said. “Movento has very good residual activity.”

Aphid management

Aphids also feast on a smorgasbord of vegetables. The most popular aphid is the green peach aphid, plus lettuce (red) aphid, foxglove aphid, potato aphid, and cabbage aphid. Luckily, all of these aphids usually appear in the field at the same time.

Green peach aphid management in desert leafy greens and cole crops can be successfully accomplished with insecticides. In Palumbo’s field trials conducted at the Yuma Agricultural Center, insecticides with the best residual control included Platinum, Assail, Admire Pro (upgraded Admire 2F), Voliam Flexi, Beleaf, and Movento.

The Leptidopterous insect complex can be a tough nut to crack with insecticides. In fall lettuce, PCAs on average treat 100 percent of the acreage two to three times per crop cycle.

Over the last several years, PCAs, growers, and Palumbo have noticed a sharp decline in beet armyworm pressure in fields in the low desert, the California Central Valley, and other vegetable-growing areas in the U.S.

Palumbo has several theories for the reduced worm numbers.

The first is the increased use of Bt (biotech) cotton seed which includes the insect-control trait Bollgard II. The trait is highly effective against the beet armyworm and corn earworm.

As part of the pink bollworm insect eradication efforts in 2008 and 2009, the percentage of acreage planted in Bt cotton in Yuma County and nearby Mexicali in Mexico’s State of Baja California increased to about 95 percent. The technology likely reduced insect levels in cotton and other nearby crops including vegetables.

Another theory is related to alfalfa. With higher alfalfa hay prices in recent years, Palumbo says alfalfa growers have invested more dollars into effective insecticides to gain higher hay yields and quality. The end result is likely fewer armyworms marching from alfalfa fields into nearby vegetables.

Yet another potential reason is the use of pyrethroids and other insecticide products in fall melons to control whitefly populations. The adult whitefly is the major vector for the virus which causes cucurbit yellow stunting disorder (CYSDV) virus which reduces melon size. The insecticides target a variety of pests including cabbage loopers.

In summary, many effective insecticides on the market control Lepidopterous larvae in desert vegetables, Palumbo says. These products include a total of eight different modes of action and more than 20 active ingredients which help reduce insecticide resistance potential over the long run.

“The take-home message is there are many highly-effective insecticides on the market today which not only provide good insect control in winter vegetables, but are environmentally friendly as well,” Palumbo concluded.